Attenuation equalizer



co STATES PATENT OFF-ICE ATTENUATION EQUALIZER Warren P. Mason, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 27, 1941, Serial No. 412,587 3 Claims. (Cl. 17844) This invention relates to wave transmission the invention, connected between two load imnetworks and more particularly to attenuation pedances; equalizers. Fig. 2 shows a modification of the circuit of An object of the invention is to equalize the Fig. 1 in which a resistor is added in series with attenuation in a wave transmission system. the piezoelectric crystal;

Another object is to compensate for attenua- Fig. 3 is a modification of the circuit of Fig. 2 tion distortion which occurs in a comparatively in which a shunt resistor is connected at the narrow percentage frequency range without inright of the series impedance branch to improve troducing appreciable distortion outside of that the impedance match at the left end of the range. In equalizer;

In a multichannel carrier telephone system, F 4 Shows a mod fication of the circuit oi for example, where a pilot channel is used in F g. 3 in, which a shunt resistor is connected at connection with automatic regulation the transthe left of the series branch to improve the immission loss in the pilot channel must be the pedance match at the opposite end; and

same as the loss in the other channels to ensure F g. 5 gives the type o quency proper operation of t e regulator, w t characteristic obtainable with the equalizer cirpilot channel is taken ofi by means of a high cuits shown in the othertfigures.

impedance shunt, including a narrow band filter, Taking up e fi u e 'in m r det i 1 connected across the transmission line attenuas ows one embodiment of the attenuation equalt distortion in t band of the pilot channel izer comprising a piezoelectric crystal element X filter is introduced. This distortion is usually and a resistor R connected n pa allel between small, extends over a comparatively narrow perterminal-S and since the unbalanced o gentage frequency range and has a maxi u is shown there iS a direct COIlllECtlOIl between value at some frequency within this range. terminals 2 d h h may be grounded or In accordance with the present invention this 95 Otherwise ed in potential. If the balanced distortion is compensated for by means f a form is desired the impedance branch between ple but efficient attenuation equalizer which has terminals l and 3 is reduced to f the Value the required inverse loss characteristic over the Shown and a Second branch of thesame mpe distortion band but has a fiat loss outside of this m is Substituted o the strap between terband so minals 2 and 4. The terminal loads are repre- The equalizer comprises a resistor associated ented by the impedance-i and one Of which with a piezoelectric crystal impedance element 13 assumed to mclude the u ce of S nal Voltage.

which has a resonance in the neighborhood of It is assumed that the attenuation distortion the maximum attenuation distortion. If the reto be corrected Qccurs between the frequencies sistor is connected in series with the transmission- I and f and has a maximum Value K at the ,7 line the crystal element is connected in shunt mtermifdlate frequency J' The t de of/ with the resistor. In order to provide the rethe reslstor 15 therefore 50 Chosen that the quired impedance for the crystal a second resistor fiat loss A mtroduced by its insettion between may be connected in series therewith, and this I the load impedance and Z2 exceeds The latter resistor may be made Variable to allow 40 crystal X is now designed to resonate at or near for manufacturing variations in the crystal and the frequency 72 and to have resistance at resonance which is low enough that the insertion to permit an adlustment of the attenuation characteristic. In order to improve the impedance loss of the parallel combmatlon o X and R 15 match between theequalizer and its terminal equal to Where loads a shunt resistor may be connected across B=AK the line at one or both ends of the series im- The crystal X is further designed t h a pedance branch. actance characteristic so steep that at the fre- The nature of the invention will be more fully quencies fl and f3 the impedance of the crystal understood from the following detailed desCripis so high that the insertion loss of the equalizer tion. and by reference to the c mpany ng dr wceases to be appreciably affected thereby. The ing, in which like reference characters refer,to equalizer will then have an attenuation charsimilar parts and in which: acteristic of the type shown in Fig. 5, with a fiat Fig. 1 is a schematic circuit of one embodiment loss A below fl and above f3 and a minimum of an attenuation equalizer in accordance with value B at f2, which will compensate for the assumed attenuation distortion. It should be pointed out that in Fig. 5 neither the frequency scale nor the, attenuation scale starts at zero.

In order to build the resistance at resonance of the crystal X out to the required value a resistor RI may be added in series therewith as shown in Fig. 2. This resistor may be made variable, as indicated, to allow for manufacturing variations in the resistance of the crystal and also to permit an adjustment of the attenuation characteristic of the equalizer. As 'the resistor RI is increased in value the minimum attenuation B of the attenuator at the frequency 12 is raised, and vice versa.

In either Fig. 1 or Fig. 2 the impedance match between the load Z! and the equalizer may be improved if the impedance of theload Z2 is so chosen that the sum of the impedance Z2 and the impedance of the series branch is equal to the impedance ZI.

. If the impedance Z2 cannot be. made smaller than Zl then an impedance match with Zl may be obtained by adding the resistor R2 in shunt with Z2, as shown in Fig. 3, to form an L-type attenuator.

The impedance of the equalizer may be matched to the load at both ends by adding another shunt resistor R3 at the left of the series impedance branch as shown in Fig. 4, to form a 1r-type attenuator.

What is claimed is:.

1. An attenuation equalizer adapted to compensate for attenuation distortion which occurs in a comparatively narrow percentage frequency range and has a maximum value at a certian frequency comprising two shunt resistors, aninterposed series resistor and a piezoelectric crystal, said crystal being connected in parallel with said series resistor and having a resonance in the neighborhood of said certain frequency.

2. An attenuation equalizer comprising two shunt resistors, an interposed series resistor and an impedance branch connected in parallel with said series resistor, said impedance branch including a piezoelectric crystal and an additional resistor connected in series. I

3. An attenuation equalizer in accordance with claim 2 in which said additional resistor is variable.

WARREN P. MASON. 

